The present invention relates to data storage systems. In particular, the present invention relates to data storage systems which utilize ferroelectric materials.
Ferroelectrics such as PbTiO3 can sustain surface charge densities up to 50 microcoulombs/cm2 at room temperature, as is noted at page 376 of xe2x80x9cIntroduction to Solid State Physicsxe2x80x9d by C. Kittel, 6th Ed., John Wiley and Sons (1986) Like ferromagnets, ferroelectric materials also can form domains with oppositely charged regions. Unlike ferromagnets, however, a ferroelectric domain wall is usually abrupt on an atomic scale, whereas a magnetic domain wall often spreads over 100 nm. This is because ferroelectrics do not have an analog of the magnetic exchange energy (see p. 193, xe2x80x9cPhysics of Crystalline Dielectricsxe2x80x9d, I. S. Zheludev, Plenum Press). In the presence of electronic and ionic polarization, the dominant interaction is that of the electrostatic forces between polarized ions. The energies of interaction for parallel and antiparallel arrays of dipoles in dielectrics are very similar and, therefore, a domain wall in a ferroelectric is much thinner than that of a magnetic domain wall. In a strongly anisotropic ferroelectric crystal, the polarization vector does not rotate within a domain wall but simply decreases in absolute value, changes sign, and recovers its original value in the opposite direction. Only in very weakly anisotropic ferroelectric material does the polarization vector actually rotate within a domain wall.
When a ferromagnet is reduced in size, the magnetic anisotropy energy decreases proportionally, while the magnetic moment is exchange-coupled into one single vector with rotational degree of freedom. This results in the so called superparamagnetic limit. For ferromagnets below this limiting volume, the thermal energy in the magnetic moment is large enough to overcome the magnetic anisotropy (which decreases with decreasing volume), and the ferromagnet would lose its average magnetic moment due to thermal fluctuation. This phenomenon is called superparamagnetism. This limiting volume of a ferromagnet is believed to set the ultimate recording density of a magnetic storage medium.
For ferroelectric ordering, the characteristic ordering energy is essentially an intensive quantity. This is believed to arise because there is no exchange coupling to bind neighboring ionic polarization sites in ferroelectric materials [see I. S. Zheludev, supra)]. Accordingly, the energy scale for electrical polarization is directly related to the atomic degrees of freedom of the ionic sites. Therefore, the ferroelectric order does not appear to suffer from the equivalent of a superparamagnetic limit. Thus in principle it may be possible to use ferroelectric domains as recording bits, and thereby attain recording densities far beyond those possible with magnetic mediums.
During the past ten years, intensive materials research in oxide thin films has greatly enriched our knowledge and ability on forming atomically smooth epitaxial oxide, especially perovskite, thin films. We have also seen much progress in single crystal substrate materials which are suitable for such thin films. Today, epitaxial, atomically smooth perovskite oxide thin films can be routinely grown on substrates of 2-3 inch in diameter with a reasonable price for small volume operation (quite possibly for less than $500/wafer for 2-inch size).
At the same time, continued miniaturization of CMOS circuits have made it possible to form FET structures with gate dimensions well below 1 xcexcm (micrometer), making it possible to fabricate small FET sensing elements with low input capacitances that matches the dimension of individual bit required for high-density read-operation.
These developments, when combined in accordance with the teachings of the present invention, gave us a unique opportunity to explore the possibility of a novel storage concept, namely a ferroelectric-based hard data storage system. In fact, recently a ferroelectric bit size of 30 nm has been experimentally demonstrated in the thin film system of PZT/SrRuO3 (see C. H. Ahn et al, Science 276, 1100 (1997) as well as Tybell et al, Appl. Phys. Lett. 72, 1454 (1998)) (Note that PZT=Pb (ZrxTi1-x) O3, where 0 less than x less than 1).
The present invention proposes to utilize these special properties of ferroelectric materials. When combined with existing magnetic hard disk""s servo-control mechanics and electronics, this approach may provide a ready alternative to magnetic storage, and increase the storage density many times over, while using device and system technologies that exists today.
The present invention broadly provides a data storage system comprising:
a) a storage medium comprising an electrically conducting substrate having a ferroelectric layer thereon, the ferroelectric layer comprising a plurality of cells at its surface, each cell comprising at least one domain, with mutually adjacent domains being capable of storing mutually opposing electrostatic charges,
b) a write head comprising an electrically conducting member comprising a projecting portion (or xe2x80x9ctipxe2x80x9d) in closely spaced adjacency to said ferroelectric layer for lateral movement relative thereto, said projecting portion being laterally smaller than a cell,
c) a read head comprising a field effect transistor (e.g. MOS FET) with its gate electrode disposed in closely spaced adjacency to said ferroelectric layer for lateral movement relative thereto, said gate electrode being laterally smaller than a cell, and
d) a drive, which may comprise an electric motor, adapted to move the storage medium laterally in closely spaced adjacency to the write head and the read head.
Preferably, the substrate is a disk that is rotatable laterally by said drive and the ferroelectric layer comprises a plurality of concentric recording tracks defined by said cells. The storage system preferably includes an elongate arm having a first end and a second end, said elongate arm being pivoted adjacent a first end to permit movement of the second end across a plurality of the concentric recording tracks, the write head and the read head being carried by said elongate arm adjacent the second end thereof
According to a preferred embodiment, the elecrically conducting substrate comprising a crystalline substrate and a conducting layer disposed on a surface thereof, while the ferroelectric layer comprises PZT, barium titanate, or lead titanate, and the conducting layer comprises a conducting oxide, such as La0.67Sr0.33MnO3 and SrRuO3.
Moreover, it is preferred that each ferroelectric storage cell has a lateral cell dimension of less than 0.3 xcexcm